FIGS. 22(a) and 22(b) are a sectional view and a perspective view, respectively, for explaining a prior art method for zone-melting recrystallization of a semiconductor film disposed on a substrate. In the figure, a semiconductor substrate 100 includes an insulating base 10 comprising a ceramic or the like. An SiO.sub.2 film 1 about 1 .mu.m thick (hereinafter referred to as a first insulating film) is disposed on the insulating base 10. A semiconductor film 2 0.5.about.1 .mu.m thick and comprising polycrystalline silicon or amorphous silicon is disposed on the first insulating film 1. A composite film B of SiO.sub.2 and Si.sub.3 N.sub.4 (hereinafter referred to as a third insulating film) is disposed on the semiconductor film 2. The third insulating film 8 is about 1.about.8 .mu.m thick. An upper carbon strip heater 4 having a width of about 2 mm and a height of about 9 mm is positioned about 1 mm above the semiconductor substrate 100. This upper strip-heater 4 moves across the substrate 100 in a prescribed direction at about 1 mm/s. The substrate 100 is disposed on a susceptor 5, and the susceptor 5 is positioned above a lower heater 6.
A description is given of the zone-melting recrystallization process.
Initially, as illustrated in FIG. 22(b), the semiconductor substrate 100 is placed on the susceptor 5, and the carbon strip heater 4 is moved across the semiconductor substrate 100 at about 1 mm/s, whereby a narrow molten zone 2a is formed in the semiconductor film 2 immediately below the strip 4. The molten zone 2a is resolidified and recrystallized when the strip 4 has passed. In place of the carbon strip heater 4, other strip heaters, such as an infrared ray lamp, may be employed.
FIGS. 23(a) and 23(b) are a sectional view and a plan view, respectively, illustrating a part of the semiconductor substrate 100 after the zone-melting recrystallization process. In the figures, the same reference numerals as in FIGS. 22(a)-22(b) designate the same or corresponding parts. After the zone-melting recrystallization, the grain size of the semiconductor film 2 is increased, whereby electrical characteristics, such as conductivity, are improved. However, as shown in FIGS. 23(a)-23(b), dislocations 2c, subgrain boundaries 2b which are caused by collected dislocations, and other defects occur in the semiconductor film 2. When a device is fabricated using the semiconductor film 2 with the subgrain boundaries 2b and the dislocations 2c, these defects act as recombination centers for carriers (electrons and holes), increasing the dark current of the device. Accordingly, in order to fabricate a device with improved characteristics, it is important to reduce the subgrain boundaries 2b and the dislocations 2c in the semiconductor film 2 or to produce the subgrain boundaries 2b at prescribed positions in the semiconductor film 2 where the subgrain boundaries do not adversely affect the device characteristics. The subgrain boundary 2b is caused by unevenness at a solid-liquid interface, i.e., an interface between the resolidified portion and the molten zone, formed in the semiconductor film 2 during the zone-melting recrystallization process.
FIG. 24(a) shows a cross-section of a semiconductor substrate 100a and FIG. 24(b) shows a temperature distribution in a semiconductor film 2 included in the substrate 100a, for explaining a zone-melting recrystallization method in which subgrain boundaries are produced at desired positions in the semiconductor film. FIG. 25 is a plan view of the semiconductor film 2. In these figures, the same reference numerals as in figures 22(a)-22(b) designate the same or corresponding parts. A plurality of stripe-shaped insulating films 7 are disposed on the insulating film 3 at prescribed intervals. Preferably, the heat insulating films 7 comprise a semiconductor material. The heat insulating films 7 are covered with an insulating film (not shown) as needed.
In this zone-melting recrystallization process, a strip heater, such as a carbon strip heater, is placed above the semiconductor substrate 100a so that the longitudinal direction of the strip heater is perpendicular to the stripe direction of the insulating films 7 and then the heater is moved across the semiconductor substrate 100a in the stripe direction of the insulating films 7 at a uniform rate. Since the stripe-shaped insulating films 7 partially shield the semiconductor substrate 100a from heat emitted from the strip heater, the temperature of portions of the semiconductor film 2 opposite the insulating films 7 is lowered, resulting in the temperature profile shown in FIG. 24(b). Therefore, the unevenness at the solid-liquid interface between the resolidified portion and the molten zone in the semiconductor film 2 is influenced by the temperature profile of FIG. 24(b), so that the subgrain boundaries are concentrated at portions of the semiconductor film 2 opposite the stripe-shaped insulating films 7, i.e., concave portions of the solid-liquid interface, as shown in FIG. 25, whereby monocrystalline regions with few defects are produced between the subgrain boundaries 2b.
However, the formation of the stripe-shaped insulating films 7 complicates the production process. In addition, it is impossible to completely eliminate the subgrain boundaries 2b from the recrystallized semiconductor film.
FIG. 26 is a plan view of a semiconductor substrate for explaining a zone-melting recrystallization method in which a semiconductor film is zone-melted and recrystallized with no subgrain boundary. In the figure, a semiconductor substrate 100b includes a base layer 10. An insulating film 1 is disposed on the base layer 10. A semiconductor film 2A comprising a plurality of island portions 2d and a plurality of line portions 2e connecting the island portions in series is disposed on the insulating film 1. The semiconductor film 2A is covered with an insulating film (not shown).
In this zone-melting recrystallization method, a strip heater, such as a carbon strip heater, is placed above the semiconductor substrate 100b so that the longitudinal direction of the strip heater is perpendicular to the longitudinal direction of the semiconductor film 2A and then the heater is moved across the semiconductor substrate 100b in the longitudinal direction of the semiconductor film 2A at a uniform rate. When a molten zone crosses an island portion 2d of the semiconductor film 2A, heat is transmitted through the line portion 2e which has already been zone-melted and recrystallized, so that the flow of heat in the island portion 2d is concentrated toward the junction between the island portion 2d and the line portion 2e. Therefore, a solid-liquid interface protruding toward the molten zone moving direction is formed in the semiconductor film 2A and the solid-liquid interface has no concave part, whereby no subgrain boundary is produced. In this method, however, when the size of the island portion 2d exceeds a certain extent, the above-described effect which depends on the shape of the semiconductor film 2A is reduced and subgrain boundaries are produced. That is, a large-sized semiconductor film with no subgrain boundary is not produced in this method. Further, since the semiconductor film 2A is formed into the island portions 2d and the line portions 2e, the utilization ratio of the semiconductor film on the substrate is low. This low utilization ratio causes serious problems when a semiconductor device including a large-sized semiconductor film, such as a solar cell, is fabricated.
FIG. 28(a) is a sectional view illustrating a conventional heater used as the lower heater 6 in FIG. 22(a), and FIG. 28(b) illustrates a profile of heat intensity applied to a substrate by the heater. In FIG. 28(a), a heater 250 includes a reflector 9 comprising a plurality of stripe-shaped concave mirrors 9a connected in parallel to each other. The reflector 9 is made of metal and the concave part 9a has an arch-shaped cross section. The internal surfaces of the concave parts 9a are plated with gold. A strip lamp 8 is disposed in the center of each concave part 9a of the reflector 9. A halogen infrared lamp is usually used as the strip lamp 8. The reflector 9 is well cooled with water so that it is not heated by the strip lamps 8. The diameter of the strip lamp 8 is about 1 cm.
Although the reflector 9 is cooled with water, the strip lamp 8 must not be disposed near the reflector 9 because the temperature of the lamp is very high. Therefore, the width .lambda. of the aperture of the concave part 9a must be at least several centimeters, so that the interval between adjacent strip lamps 8 is several centimeters.
When the semiconductor substrate 100 is heated by the heater 250, the optical path length of the radiation (heat ray) from the strip lamp 8 directly to the semiconductor substrate 100 is different from the optical path length of the radiation (heat ray) reflected by the reflector 9 before reaching the substrate 100. Further, the radiation reflected by the reflector 9 is weakened in accordance with the reflectance, so that the intensity of heat applied to the semiconductor substrate 100 by the heater 250 is periodically lowered with a period corresponding to the aperture width .lambda. of the concave part of the reflector 9 as shown in FIG. 28(b). Accordingly, the heater 250 employed as a lower heater in this zone-melting recrystallization process does not heat the entire surface of the semiconductor substrate 100 with uniform heat intensity, whereby unmelted regions remain in the semiconductor film of the semiconductor substrate 100.
When a semiconductor film is subjected to zone-melting recrystallization, the uniformity of the crystallinity of the recrystallized semiconductor film is improved as the width of the molten zone formed in the semiconductor film is reduced. Therefore, reduction in the width of a carbon strip heater that is conventionally used as an upper heater has been attempted. However, since the carbon strip heater is usually supported by a movable member, if the carbon strip heater is thinner than 2 mm, the heater may be broken when it emits heat and expands.
Likewise, in order to improve the crystallinity of the semiconductor film in the zone-melting recrystallization process, it is desired that the temperature of the molten zone fall gently. FIG. 27 illustrates a temperature distribution in the vicinity of the carbon strip heater 4. If minute control of this temperature distribution is possible, the crystal quality of the semiconductor film is further improved. However, the temperature distribution in the vicinity of the heater 4 is unconditionally determined by the size and sectional form of the heater and the atmosphere surrounding the heater, so that it is impossible to minutely control the temperature distribution.
Furthermore, in the conventional apparatus for zone-melting recrystallization, an advanced machine control technique is required to move the high temperature heating element, i.e., the carbon strip heater, across the semiconductor substrate in a prescribed direction at a uniform rate while maintaining a prescribed distance from the substrate. Therefore, it is difficult to perform that operation with high reliability and repeatability.
As described above, the zone-melting recrystallization methods shown in FIGS. 24(a)-24(b) and 26 can limit the subgrain boundaries within prescribed regions in the semiconductor film or reduce the subgrain boundaries and dislocations in the semiconductor film. However, the formation of the heat insulating films 7 (FIG. 24(a)) or the patterning of the semiconductor film before the zone-melting recrystallization process (FIG. 26) complicates production. In addition, as already described with respect to FIG. 26, there is a limit in the size of a monocrystalline region with no subgrain boundaries produced by the conventional method. Therefore, a large-area monocrystalline semiconductor film with no subgrain boundaries has not yet been released.
Furthermore, the lower heater generally used in the zone-melting recrystallization process cannot heat the target (semiconductor substrate) uniformly because of its structure, so that unmelted portions unfavorably remain in the semiconductor film during the zone melting recrystallization process.